It could be too late to get a rail system to the airport, a Campaign for Better Transport spokesman says.

Cameron Pitches says the planned widening of State Highway 20A and the massive number of buildings erected around the site might have scuppered any chance of building an "L" shaped rail line from Onehunga to the airport and then to Manukau. The project is still being investigated and he has told Auckland Council's infrastructure committee about his concerns.

"In 2007 we presented a petition with more than 100,000 signatures in support of rail to the airport," he says.

"Since then there have been new buildings built that are taking up space where rail might go.

"And with the Government planning on widening the road to the airport, this has taken up more space that could have been used for rail.

"The rail services would be primarily servicing the workers . . . public transport in South Auckland is particularly bad."

The number of people going to the airport is expected to rise over the next 30 years from the current 63,000 to 140,000 a day.

But Auckland Transport says it's still working with the airport on how transport links can be improved.

Rail is the best option to meet increasing demand and a preferred alignment for a rail line is expected to be decided this year so it can go out for public consultation, it says.

Airport company chief executive Adrian Littlewood says enough land has been set aside for rail along with space for an underground station. He says space has also been created for new busways to and from the airport.

The number of people flying in and out is expected to grow to 40 million a year, three times the current number.

Pitches says Auckland would be a much better host to international tourists if they could get on a train after arriving in the country.

"After a long flight tourists are quite limited with how they can get into the city. They have to hire a car or get a taxi and we all know how expensive that can be.

"I think most overseas visitors expect a rail service to get them back to the city - it's pretty standard in a big city."

The Government earmarked $140 million in this year's budget for widening State Highway 20A between Mangere and the airport.

Pitches says that shows it does not have public transport in mind.

"Instead of building roads and then figuring out where public transport can go, we should be doing it the other way around.